1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) for displaying a gray-scale image, and particularly, to an LCD that displays a gray-scale image according to image data quantized by an analog-to-digital (A-D) converter.
2. Description of the Related Art
Television sets employing LCDs usually have A-D converters to digitize analog image signals. A simple-matrix LCD for displaying dynamic images converts an analog luminance signal into a digital signal and carries out pulse-width modulation on the digital signal. The same technique is applicable to an active matrix LCD employing MIM elements, as disclosed in Japanese Examined Patent Publication No. 63-6855. To reduce costs, the LCD television sets frequently employ low-number-of-bit A-D converters and a dithering technique to provide gradations.
FIG. 7 is a block diagram showing an LCD, according to a prior art, employing the dithering technique. The LCD has a 4-bit A-D converter 906. An image signal 902 is supplied to an analog input terminal AIN of the A-D converter 906, which converts the signal into 4-bit data. The 4-bit data is passed through a data bus 908 and is stored in a memory 910a of a signal electrode driver 910. A multiplexer 904 has a top switch 904a for switching high reference voltages Vt1 and Vt2 from one to another and a bottom switch 904b for switching low reference voltages Vb1 and Vb2 from one to another. The output of the switch 904a is connected to a high-reference-voltage input terminal Vrt of the A-D converter 906, and the output of the switch 904b is connected to a low-reference-voltage input terminal Vrb of the converter. The switches 904a and 904b are interlocked and controlled in response to a clock signal .PHI.1 provided by a controller 916.
The controller 916 provides the A-D converter 906 and driver 910 with a signal group .PHI.2 generated according to a horizontal synchronous signal. The signal group .PHI.2 includes a data sampling clock signal, a shift clock signal used to prepare an address of the memory 910a, a latch clock signal used to transfer data in the driver 910, and a timing signal used to carry out pulse-width modulation. The controller 916 provides a scanning electrode driver 912 with a signal group .PHI.3, which includes vertical scan signals such as a start signal for starting a scan operation and a clock signal for sequentially shifting a select pulse. The driver 910 consists of the memory 910a and a pulse-width modulator 910b. The output terminals of the driver 910 are connected to the signal electrodes of a liquid crystal panel 914, respectively. The output terminals of the driver 912 are connected to the scanning electrodes of the panel 914, respectively.
The memory 910a successively stores 4-bit data for one horizontal scan period and then transfers all stored data to the pulse-width modulator 910b. According to a line at a time scanning method generally used, the A-D converter 906 successively quantizes the image signal 902 for a first horizontal scan period into 4-bit data. The 4-bit data is stored in the memory 910a. In a second horizontal scan period, the 4-bit data for the first horizontal scan period is transferred to the modulator 910b in response to the latch clock signal of the signal group .PHI.2 provided by the controller 916. The modulator 910b carries out pulse-width modulation on the 4-bit data and provides drive signals to the signal electrodes of the panel 914. At this time, the driver 912 provides a select signal to a corresponding scanning electrode. As a result, a LCD driving voltage is applied to each pixel in the selected scan line to display a gradation. In the mean time, the A-D converter 906 successively quantizes the image signal 902 for the second horizontal scan period into 4-bit data, and the 4-bit data is stored in the memory 910a. These processes are repeated to display an image on the panel 914.
A dithering technique of realizing 32 gradations with the 4-bit A-D converter 906 will be explained. The image signal 902 supplied to the A-D converter 906 is, for example, a raster signal having a uniform luminance. The liquid crystal panel 914 has, for example, 240 scanning electrodes and is driven according to an NTSC method. The reference voltages are set as follows: EQU Vt1&lt;Vt2 EQU Vb1&lt;Vb2
According to the NTSC method, the panel 914 is scanned from the top to the bottom in a period called a "field." The panel 914 is wholly scanned by two fields that form a "frame." In the first field, the panel 914 is scanned on every other line, and in the second field, on the remaining lines. The dithering technique expresses a gradation with a plurality of fields, and therefore, it halves the vertical resolution of an image.
In the first field, a combination of the high and low reference voltages Vt1 and Vb1 is used for converting an analog raster signal into digital data having a gradation "n," and voltages corresponding to the data are applied to pixels of the panel 914. In the second field, a combination of the high and low reference voltages Vt2 and Vb2 is used by the A-D converter 906 to convert the raster signal into digital data having a gradation "n" or "n-1." If the first and second fields display the gradation n, they are viewed as the gradation n. If the first field displays the gradation n and the second field the gradation n-1, they are averaged in human eyes and are viewed as a gradation "n-0.5." In this way, 0.5-interval gradations are realized, and the 4-bit A-D converter 906 achieves 32 gradations.
According to this prior art, the clock signal .PHI.1 for switching the reference voltage pairs from one to another alternates between high and low levels field for each field. When the polarity of a LCD driving voltage applied to one scanning electrode is opposite to the polarity of a LCD driving voltage applied to the adjacent scanning electrode and when the polarity of a LCD driving voltage applied to a given scanning electrode is inverted field by field, the period of inversion of the polarity of the LCD driving voltage agrees with the period of inversion of the clock signal .PHI.1. Consequently, the same reference voltage pair is always selected for any one of the polarities of the LCD driving voltage. When a raster signal is converted into two pieces of data having individual gradations that are different from each other by one level for the first and second fields, a LCD driving voltage applied to a given scanning electrode or a given pixel differs depending on the polarity of the LCD driving voltage. The difference is always in the same direction if the period of inversion of the polarity of the LCD driving voltage is the same as the period of inversion of the clock signal .PHI.1. This results in applying a direct-current voltage to the panel 914 to stick images into the panel.